By forming a mutual relationship with the host, the vaginal microbiome (i.e. various kinds of vaginal microbial communities present in healthy women) has a significant impact on women’s health and disease. In the recent years our knowledge of those vaginal bacteria communities has expanded rapidly as a consequence of using modern molecular (cultivation-independent) methods for species identification.

Bacterial vaginosis represents the most prevalent type of vaginal infection that occurs among reproductive-age women, and also the most common condition that prompts women to seek medical care. This condition, which is highly dependent on resident microbial flora, is linked to some severe sequelae such as pelvic inflammatory disease, preterm labor, as well as increased sensitivity to infection with various sexually-transmitted agents.

A Shift in Vaginal Microbial Communities

Although the term ‘bacterial vaginosis’ was coined more than half a century ago, its etiology and natural history remains elusive. Generally, this condition is characterized by a shift in microbial composition from an abundance of lactobacilli in healthy women, to an increase of commensal anaerobic bacteria (such as Gardnerella, Atopobium, Prevotella and a panoply of other species).

As the aforementioned loss of lactobacilli results in reduced lactic acid production, the pH in the vaginal lumen increases. Furthermore, amine and salidase production increases, while the production of hydrogen peroxide and lactocin is diminished, leading to signs and symptoms of bacterial vaginosis.
 

Women with this condition are characterized by heterogeneous communities of bacteria that exhibit increased richness and diversity of existent bacterial species. Such species heterogeneity in bacterial vaginosis is thought to arise from functional redundancy between them, which is in turn associated with improved community reliability when faced with environmental changes.

Moreover, a thorough analysis of the vaginal microbiome may predict the recurrence of bacterial vaginosis if changes in the resident microflora are compared. Such recurrences in sexually-active women are not rare, and may even appear three times or more during the year.

Microbial Associations with Clinical Criteria for Bacterial Vaginosis

In a majority of clinical settings, women are diagnosed with bacterial vaginosis by using the Amsel criteria when three or more signs are present, such as a vaginal pH higher than 4.5, thin vaginal discharge, amine odor if potassium hydroxide is added to vaginal fluid, as well as the presence of vaginal epithelial cells overlaid with bacteria (also known as “clue cells”).

Nugent scores that are based on weighted tallies of various cellular morphotypes (most notably lactobacilli, Gardnerella vaginalis, Bacteroides, as well as curved Gram-variable rod-shaped bacteria) are also widely used in the diagnosis of bacterial vaginosis. This scoring system ranges from 0-10, and scores higher than 7 are indicative of bacterial vaginosis.

The advantage of these clinical criteria is that the diagnosis can be achieved by using light microscopy; however, that way it is impossible to get deep insights into the composition of present bacterial communities. On the other hand, deep sequencing analysis is quite expensive and still not pervasive in clinical practice, even though it can provide a clear picture of vaginal microbiota and allow the detection of recurrences and treatment failures.